1. Field of the Invention
The present invention relates to an axial symmetric polarizing plate, a method for fabricating the axial symmetric polarizing plate, and a liquid crystal display device using the polarizing plate. More Specifically, the present invention relates to an axial symmetric polarizing plate having light transmission easy axes or light absorption axes arranged with axial symmetry so that the light transmission or light absorption of the polarizing plate for incident light is uniform omnidirectionally in one plane, and a method for easy fabrication of such an axis symmetric polarizing plate.
The present invention also relates to a liquid crystal display device where the viewing angle characteristics are uniform omnidirectionally by combining the above axial symmetric polarizing plate and the orientation of liquid crystal molecules of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device having wide viewing angle characteristics is suitable, not only for personal display apparatuses such as wordprocessors and personal computers, but also for apparatuses such as portable information terminals which are viewed by a plurality of users, especially one which is viewed by several users gathering in front of the apparatus on a desk. This type of liquid crystal display device should be used both as a reflective liquid crystal device and a portable display device. Such a liquid crystal display device is now under development.
Hereinbelow, the principle for the improvement in the viewing angle characteristics of a liquid crystal display devise will be described.
FIGS. 24A to 24C illustrate the change in the orientation of liquid crystal molecules in a liquid crystal cell where the liquid crystal molecules are oriented in two different directions. FIGS. 24D to 24F illustrate the change in the orientation of liquid crystal molecules in a liquid crystal cell of a conventional twisted nematic (TN) mode,
Referring to FIGS. 24A to C, a liquid crystal cell 11 corresponding to one pixel includes upper end lower substrates 2 and 1 facing each other and a liquid crystal layer formed therebetween. The liquid crystal cell 11 has two liquid crystal domains 8a and 8b where liquid crystal molecules 9 are initially oriented in different directions. The liquid crystal domains 8a and 8b are surrounded by a partition 7 made of polymer and the like. The reference numeral 10 denotes the boundary between the liquid crystal domains 8a and 8b. Referring to FIGS. 24D to 24F, a liquid crystal cell 12 corresponding to one pixel is of the TN mode and includes liquid crystal molecules 13 oriented in one direction. Upper and lower polarizing plates (not shown) are disposed on the upper and lower substrates 2 and 1 of both the liquid crystal cells 11 and 12 so that the polarization axes thereof are perpendicular to each other (normally white mode).
In order to improve the viewing angle characteristics of a liquid crystal display device, it is necessary to form two or more domains where liquid crystal molecules are oriented in different directions in each liquid crystal cell corresponding to one pixel as shown in FIGS. 24A to 24C.
The reason is that, as shown in FIG. 24B, in a gray-scale state of the liquid crystal cell 11, i.e., in the state where the liquid crystal molecules are in transition between the horizontal orientation and the vertical orientation by the application of a voltage, the transmittances for light incident from directions A and B are averaged, and thus the same contrast ratio is obtained in the directions A and B. As a result, the viewing angle characteristics of the liquid crystal cell 11 in the gray-scale state are improved compared with that of the liquid crystal cell 12 of the TN mode.
However, in a saturated voltage application state of the liquid crystal cells 11 and 12 (FIGS. 24C and 24F), the liquid crystal molecules 9 and 13 are oriented along the electric field. Therefore, in the saturated voltage application state, the orientations of the liquid crystal molecules are the same in the liquid crystal cells 11 and 12 though they are initially different. In the saturated voltage application state, as for the optical characteristics in a direction inclined from the normal of the liquid crystal display device, a region where the viewing angle characteristics are comparatively poor is generated in the 45.degree. directions from the polarization axes of the upper and lower polarizing plates. This is due to the synergistic effect of the viewing angle characteristics of the upper and lower polarizing plates and the light leakage by an elliptically polarized light component caused by retardation generated as light passes obliquely through the liquid crystal layer. Even a liquid crystal display device, including the liquid crystal cell 11 which has two domains where liquid crystal molecules are oriented in different directions, does not have omnidirectional viewing angle characteristics since the device itself is anisotropic for the viewing angle.
In other words, the problem that the display performance is comparatively poor in 45.degree. directions from the polarization axes of the upper and lower polarizing plates arises because the polarization axes of the polarizing plates are arranged in a predetermined direction over the entire liquid crystal cell.
Next, some examples of conventional liquid crystal display devices of a wide viewing angle mode will be described.
(1) A method for electrically controlling the transparent state or the opaque state of a liquid crystal device using birefringence of liquid crystal molecules in a liquid crystal cell having polymer walls has been proposed. In this method, basically, the ordinary light refractive index of the liquid crystal molecules and the refractive index of a medium supporting the liquid crystal are set to be equal to each other. When a voltage is applied, the liquid crystal molecules become oriented in such a way as to allow the transparent state to be displayed. When no voltage is applied, the liquid crystal molecules become oriented in such a way as to allow the light scattering state to be displayed.
Japanese Laid-Open Patent Publication No. 61-502128 concretely discloses one such method. According to this method, a liquid crystal material and a photocurable or thermosetting resin are mixed. The liquid crystal material is deposited by curing the resin, forming liquid crystal drops in the resin. In this method, neither the polarizing plate nor the alignment treatment for liquid crystal molecules is required.
Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928 disclose a wide viewing angle mode where the above liquid crystal device is combined with polarizing plates perpendicular to each other (crossed Nicols state).
More specifically, Japanese Laid-Open Patent Publication No. 4-338923 discloses a liquid crystal display device where a liquid crystal layer of a liquid crystal cell includes a liquid crystal medium, and liquid crystal capsules dispersed in the liquid crystal medium. Two polarizing plates having polarization axes perpendicular to each other are disposed on the opposite surfaces of the liquid crystal cell. A phase plate having a disk shape refractive index anisotropy is formed between the liquid crystal cell and one of the polarizing plates. In this type of liquid crystal display device, display is conducted by switching between whether light is scattered or not scattered by liquid crystal molecules in the liquid crystal capsules. Thus, the viewing angle dependence seldom occurs.
Japanese Laid-Open Patent Publication No. 4-212928 discloses a liquid crystal display device where a liquid crystal panel is constructed so that the orientation of liquid crystal molecules is disordered to scatter incident light when no voltage is applied, and that the incident light is allowed to pass through when a voltage is applied. A pair of polarizing plates having polarizing directions perpendicular to each other are disposed on the opposite surfaces of the liquid crystal panel. A disk shape optical anisotropy film is formed between the liquid crystal panel and one of the polarizing plates.
(2) Japanese Laid-Open Patent Publication No. 5-27242 discloses a method for forming a composite material layer composed of a liquid crystal material and a polymer material as a liquid crystal layer of a liquid crystal cell. The composite material layer is formed by phase separation of the liquid crystal material and the photocurable resin from the mixture thereof. According to this method, the orientation of liquid crystal molecules in liquid crystal domains is disordered by the existence of the resultant polymer portions, and thus the directions in which the liquid crystal molecules in the respective liquid crystal domains rise when a voltage is applied are different. As a result, the apparent refractive indexes observed in different directions are the same, allowing the viewing angle characteristics in the gray-scale state to be improved.
(3) Japanese Laid-Open Patent Publication No. 6-301015 by the inventors of the present invention discloses a liquid crystal display device having significantly improved viewing angle characteristics. In this liquid crystal display device, liquid crystal molecules are aligned uniformly omnidirectionally (spirally, for example) in one pixel region by controlling the intensity of illuminated light using a photomask and the like at photo-polymerization. Such liquid crystal molecules oriented in the spiral shape show substantially the same behavior as liquid crystal molecules of the TN mode in liquid crystal regions having various orientations by controlling them with voltage application. This significantly improves the viewing angle characteristics.
(4) Japanese Laid-Open Patent Publication No. 5-96289 by the inventors of the present invention discloses a liquid crystal display device where alignment films are composed of a crystalline polymer having a spherulite or spherulite-like structure. With this structure, the wide viewing angle display mode is realized since the spherulite structure can control the axial symmetric orientation of liquid crystal molecules through the liquid crystal display.
(5) Japanese Laid-Open Patent Publication No. 6-194655 discloses a method for aligning liquid crystal molecules in random directions using alignment films which are not subjected to any alignment treatment such as rubbing formed on substrates.
(6) Japanese Laid-Open Patent Publication No. 6-265902 discloses a liquid crystal display device where one of substrates of a liquid crystal cell is subjected to alignment treatment so that the orientation of liquid crystal molecules per pixel is concentric, while the other substrate of the liquid crystal cell is subjected to alignment treatment so that the orientation of the liquid crystal molecules is radial. In this liquid crystal display device, the liquid crystal molecules rise symmetrically omnidirectionally when a voltage is applied. The same display quality can be obtained when the display screen is viewed in any direction. The technique disclosed in this publication actually leads to the improvement in the viewing angle characteristics. However, since a pair of polarizing plates of which polarization axes are perpendicular to each other are used, the liquid crystal molecules are oriented along the electric field in the saturated voltage application state. Accordingly, the liquid crystal display device has the anisotropy of the viewing angle characteristics where a region in which the viewing angle characteristics are comparatively poor is generated in the 45.degree. directions from the polarization axes of the polarizing plates.
Next, liquid crystal display devices where the viewing angle characteristics have been improved by the combination of an axial symmetric polarizing plate and the axial symmetric orientation of liquid crystal molecules.
As a method for arranging polarization axes with axial symmetry, Japanese Laid-Open Patent Publication No. 60-256120 discloses a method for infiltrating a dichroic dye into a substrate subjected to rotary rubbing. Japanese Laid-Open Patent Publication No. 6-324337 discloses a method where a dichroic dye is adsorbed by or infiltrated into a substrate along numerous concentric or radial narrow grooves formed on the substrate.
In the above methods, however, the orientation of dichroic dye molecules cannot be significantly controlled by the adsorption, infiltration, and the like, so that the contrast of the polarizing plate is low.
Japanese Laid-Open Patent Publication No. 6-324337 discloses a method where liquid crystal molecules are aligned along narrow grooves formed concentrically or radially on a surface of a substrate which also serves as an axial symmetric polarizing plate as that described above.
In the above method, however, the narrow grooves are not so effective in aligning the liquid crystal molecules compared with the normal rubbing method. Therefore, the orientation of the liquid crystal molecules may be changed by voltage application. Also, since the narrow grooves are formed by lithography, the control of pretilt is difficult. This makes it difficult to reduce disclination (light leakage) generated in the peripheries of pixels.
Next, the improvement in the viewing angle characteristics in the 45.degree. directions using a phase plate will be described.
The reason why the viewing angle characteristics in the 45.degree. directions from the polarization axes of the polarizing plates is that, in these directions, light is observed as elliptically polarized light after it has passed through the liquid crystal layer where liquid crystal molecules are oriented vertically with respect to the liquid crystal cell when a voltage is applied.
In other words, since liquid crystal molecules have refractive index anisotropy (birefringence), light incident obliquely on a surface of the liquid crystal cell passes obliquely through a refractive index ellipsoid of the liquid crystal molecules oriented vertically. As for a light ray passing in a 45.degree. direction from the polarization axis of the polarizing plate, the direction of the major axis of the ellipsoid which is the intersection of the plane of which normal is the optic axis of the light ray and which includes the center of the refractive index ellipsoid and the refractive index is not the same a the direction of the polarization axis of the polarizing plate on the surface of the liquid crystal cell. Moreover, the phases of light propagating along the major axis and minor axis of the ellipsoid are different from each other. Accordingly, light incident in a 45.degree. direction from the polarization axis of the polarizing plate becomes linearly polarized light after passing through the polarizing light on the incident side, and then becomes elliptically polarized light when it passes through the liquid crystal layer, reaching the polarizing plate on the emergent side as the elliptically polarized light. As a result, the transmitted light includes a polarized light component in the direction of the polarization axis of the polarizing plate on the emergent side. Thus, light leakage occurs.
In order to overcome this problem, Japanese Laid-Open Patent Publication No. 4-212928 discloses disposing a phase plate having a disk shape negative anisotropy between the liquid crystal layer and the polarizing plate to compensate the birefringence (refractive index anisotropy) of liquid crystal molecules generated when a voltage is applied.
In this method, however, if the difference in the refractive index between the major-axis direction (z direction) and the minor-axis direction (x and y directions) of liquid crystal molecules is made larger to reduce the birefringence of the liquid crystal molecules, coloring is observed in the viewing angle characteristics when no voltage is applied. This method is therefore not practical. When the birefringence is reduced, the effect of improving the viewing angle is smell.
The above conventional methods have following disadvantages.
(1) In the axial symmetric polarizing plate configured so that the light transmission easy axes or light absorption axes are axial-symmetric (concentric, radial, or spiral) for every pixel, the orientation of the dichroic dye molecules cannot be significantly controlled, so that the contrast as the polarizing plate is low.
In the conventional axial symmetric polarizing plate, polarizing elements with different orientations are required on upper and lower substrates. This increases production cost.
(2) In the liquid crystal display device where concentric or radial narrow grooves are formed for every pixel, and liquid crystal molecules are oriented along the narrow grooves; the pretilt angle cannot be controlled, disclination lines are generated between adjacent pixels when a voltage is applied, and the contrast is lowered.
(3) The above liquid crystal display device is susceptible to an external force, generating display variation when the display screen is pushed, and thus is not suitable as a pen input device.